One rep maximum (one repetition maximum or 1RM) in weight training is the maximum amount of weight one can lift in a single repetition for a given exercise. One repetition maximum can be used for determining an individual's maximum strength and is the method for determining the winner in events such as powerlifting and weightlifting competitions. One repetition maximum can also be used as an upper limit, in order to determine the desired "load" for an exercise (as a percentage of the 1RM). SOURCE

Why do you need to know your 1 rep max? (Also referred to as PR's)

1. To gauge your current strength level

2. Some workout programs call for you to lift 65-95% of your max

3. Gives you a lifting goal, something to shoot for which makes training a challenge and keeps it interesting.

Breaking your one rep max or personal records is a great motivator, and it's also a great way to add some excitement into your training.

I have a dry erase board in my gym that I use to keep track of my PR's.

One way to find your one rep max is by using this formula:

Weight x Reps x .0333 + Weight = Estimated 1RM

Example: Say you can bench press 135lbs for ten reps; the math looks like this: